In this article I discuss a big frustration electrical engineers and electrical hobbyist face.

Why is it, that for simple circuits, the measurements are completely different from calculations!!! Infact when writing my article on negative resistance, as you would have seen - it happened again - along with the frustration :|

In my negative resistance circuit, the source voltage supplied by my wave generator was set to 4V. The measured voltage when the generator was attached to the rest of the circuit was however 1.6V
Soooo.... what is the source of the frustration? - Well we never question our tools! - especially when they are some how digital based!

My theory is, we have been taught to think: In the digital world, there are plenty of opamps and transistors. And like transistor circuits and opamp circuits - if a device's output should have low resistance, for example a wave generator, its resistance would be extra low. If a device's input should have high resistance, for example an oscilloscope, its resistance would be extra high!
Now I'm not claiming this concept is false. However here is what to do after you confirm your calculations and connections are 100% correct. First - make sure your probes from your oscilloscope is set to the highest resistance (I set mine to ×10) and your probes from the wave generator is set to the lowest resistance (I set mine to ×1).

We test again!

Wow - look at that! The output shot up to 44V! - from 4V to 44V!

And now the input...

...mmm no improvement.


Number two! The next thing you can do to decrease output resistance (or increase input resistance) is to buffer the input or the output.

Alright, we start by only buffering the input.


Above as you can see, I have placed a voltage buffer at the input.

Here is what the negative resistance circuit looks like with its input voltage buffer!


We measure our input voltage and...

3.9V! Our source voltage is less than 3% of our wave generator set voltage!

But can we call this a fix? We haven't seen how this addition affects the overall circuit's performance! So let's place the measuring probe at the output as shown below.


And the results:

...hmmmm not bad! 76V - from 4V to 76V - getting bigger!

So this ends my article on a souce of frustration for electrical engineers, along with how to avoid it. Question your instrument's resistance! And adjust your instruments settings to get maximum resistance or minimum resistance if needed! Use voltage buffers if all else fails.

...remember I said you will be impressed with how high we can make the voltage skyrocket? continue reading...
Now in the negative resistance circuit, we used a voltage divider equation [Vout=R1/(R1+R2)Vin]. We made R1 capacitive reactance, -1/(2πfC) [C=560pF], and R2 inductive reactance, 2πfL [L=39mH]. We equated the two and found f = 34056hz. This theoretically made the denominator 0 - and everybody knows what happens when you divide by 0! Sky-rocketing output!

But hold on...capacitors and inductors have tolerance levels! Infact very often, a tolerance level of 10%. So this means there is a chance that the frequency f we solved for will not make our denominator 0 - close, but not 0! To find what range the optimal frequency should be within, let's find all f for the capacitor and inductor at +10% and -10% stated value.
So from the above table we see the optimal value should lie within 33.3kHz and 34.8kHz

And the result!
WOW!!! 124V! From 4V to 124V! That is more than 30 times! Now that is impressive!



2 Important Side Notes to keep in mind.

There were 100 ohms of DC resistance measured in this inductor. This and other modes of energy loss in an inductor will now limit how much higher your peak can be.

Also, sometimes your peak voltage might not lie within this +/-10% range. Remember some components can have much less impressive tolerance levels. Furthermore, depending on the age and (improper)use of your components, you may find that your component's capacitance or inductance may exceed its rated tolerance levels.

Inductor Capacitor Oscillator Caculator Applet


This applet is digitally signed by digicert.com and works like a charm (Last checked: 24 July, 2015). Simply enter two values and leave blank the quantity you want to find. Enjoy :)

Click Start, then the Load Preset drop down menu and search for LC Resonance Applet. Select the tab at the bottom of the applet labeled 0.



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